Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Thus, a wide variety of medical conditions and disorders have been successfully treated using implanted stimulators. Such stimulators will typically stimulate internal tissue, such as a nerve, muscle, organ, or the like, by emitting an electrical stimulation current according to programmed stimulation parameters.
One class of such implantable stimulators are typically characterized by a small, cylindrical or rounded-corner rectangular housing that contains electronic circuitry that produces the desired electric stimulation current between spaced electrodes that are disposed on the external surface of the stimulator housing. These stimulators, also referred to as microstimulators, are implanted proximate to the target tissue so that the stimulation current produced by the electrodes stimulates the target tissue to reduce symptoms or otherwise provide therapy for a wide variety of conditions and disorders.
In one embodiment, illustrated in FIGS. 1A and 1B, a microstimulator 14 has a housing 15 that generally includes a battery 18, a tube assembly 20 that houses the active electronic circuitry, and a feed-through assembly 22. Electrodes 16 are affixed to the external surface of the housing 15. A more detailed description of the components of the microstimulator 14 may be found in U.S. Patent Application Publication No. 2007/0112404, which is hereby expressly incorporated by reference. Since the electrodes 16 are disposed on the external surface of the microstimulator 14, it is necessary to position the microstimulator 14 directly over a target tissue site, e.g., a target nerve 12. Further, due to structural anatomical limitations (e.g., in the ankle for tibial nerve stimulation), it is often necessary to position the microstimulator 14 parallel (as shown in FIG. 1A), rather than perpendicular, to the target site 12. Significantly, proper location and maintenance of the microstimulator position are crucial in order to continuously achieve efficacious therapy. If the position of the microstimulator 14 shifts away from the target stimulation site 12 due to migration for example, as depicted in FIG. 1B, it is possible that the patient will receive little benefit from the implanted microstimulator 14. Thus, the effectiveness of the implanted microstimulator 14 may be significantly decreased due to migration. It is therefore important that the microstimulator 14 be accurately located at the target site 12 and that the microstimulator 14 be securely maintained at the target site 12.
However, it is difficult to avoid migration of the implanted microstimulator. There thus remains a need for a microstimulator that can be positioned adjacent to a target stimulation site, yet can still operate effectively if migration occurs.